The B2 piston is probably stuck / seize on the aluminum bushing. Try tapping the piston in to free it up. If that does not work, the valve body has to be removed and some persuasion used. Once the valve body is removed you can easily remove the seal and bushing.

If you can not remove the piston seal from the exterior easily, then with a screwdriver as a chisel crush the seal into its center it should pop out then.